Linear relation between H i circular velocity and stellar velocity dispersion in early-type galaxies, and slope of the density profiles

Abstract

We report a tight linear relation between the H i circular velocity measured at 6 Re and the stellar velocity dispersion measured within 1 Re for a sample of 16 early-type galaxies with stellar mass between 1010 and 1011 M⊙. The key difference from previous studies is that we only use spatially resolved vcirc(H i) measurements obtained at large radius for a sizeable sample of objects. We can therefore link a kinematical tracer of the gravitational potential in the dark-matter dominated outer regions of galaxies with one in the inner regions, where baryons control the distribution of mass. We find that vcirc(H i)= 1.33 σe with an observed scatter of just 12 per cent. This indicates a strong coupling between luminous and dark matter from the inner- to the outer regions of early-type galaxies, analogous to the situation in spirals and dwarf irregulars. The vcirc(H i)-σe relation is shallower than those based on vcirc measurements obtained from stellar kinematics and modelling at smaller radius, implying that vcirc declines with radius - as in bulge-dominated spirals. Indeed, the value of vcirc(H i) is typically 25 per cent lower than the maximum vcirc derived at ~0.2 Re from dynamical models. Under the assumption of power-law total density profiles ρ ∝ r-γ, our data imply an average logarithmic slope 〈γ〉 = 2.18 ± 0.03 across the sample, with a scatter of 0.11 around this value. The average slope and scatter agree with recent results obtained from stellar kinematics alone for a different sample of early-type galaxies.